This invention relates to methods and apparatus for determining the concentration of various components, in particular oxygen (in the form of oxygen-containing compounds, or oxygenates), in hydrocarbon fuel mixtures containing, e.g., gasoline.
Considerable attention has been directed over the past several decades to the use of alcohol and alcohol/gasoline mixtures as fuel for automobiles. For example, retrofittable hardware has been developed to enable vehicles to run on pure alcohol or on an alcohol/gasoline mixture. Because adjustment of the carburetor and/or fuel warm up system is generally necessary to accommodate differences in fuel composition, there is a need for some type of monitor means able to determine accurately (and, preferably, on a substantially continuous basis) the alcohol and gasoline content of a mixture over the entire range of possible compositions (i.e., 100% alcohol to 100% gasoline).
In addition, it would be desirable to have some means for rapid and accurate determination of oxygenate quantity in gasolines for purposes of quality control and service station monitoring. Ideally, such a system would not only measure the dilution of gasoline with oxygenates, but also differentiate among the various oxygenate components (including, in particular, alcohols and water) and take into account variations in the composition of the hydrocarbon component.
Further, it would be advantageous to be able to determine accurately the fuel value of various gasoline compositions. A major portion of the fuel value of these compositions is derived from the aliphatic hydrocarbon (i.e., gasoline) components of the mixture. An accurate assessment of fuel value, however, would also take the amount of aromatic components of the mixture (e.g., benzene, toluene, xylenes, etc.) into account. Moreover, while many oxygenates have some fuel value, the heretofore available methods generally do not distinguish between such oxygenates and water, which has no fuel value.
Typically, finished gasoline is prepared from a variety of "blending stocks" which are combined to provide a product having the desired octane rating. Such blending stocks which are well known in the art include hydrocrackate, FCCU gasoline (regular unleaded), reformate (high octane unleaded), natural gasoline ("straight run"), coker gasoline and alkylate. Whereas hydrocrackates and alkylates are substantially free of aromatics, reformates may contain some aromatics and coker gasolines are generally rich in them. Normally, it is desirable to have a variety of stocks available for blending to provide a finished gasoline product, in view of synergies among the various types of stock with respect to octane rating; in general, the octane rating goes up with more complex mixtures. Such blending is an important aspect of the profitability of the operations of a refinery.
There have heretofore been disclosed systems for determining the alcohol/gasoline ratio in fuels by optical transducer means [van der Weide, J. et al., "A Retrofittable Alcohol/Petrol Carburation System," Paper B-25, 4th International Symposium on Alcohol Fuels, Brazil, pages 379-383 (1982)]. This type of system exploits differences between the refractive index of the liquid being evaluated and that of a permanent conductor (for example, a glass rod) inserted into the liquid.
Where the refractive index of the permanent conductor is different from that of the surrounding liquid medium, there will be a certain amount of reflection of light at the border surface of the two media. With a determination that a light source (for example, a light emitting diode or LED) has an essentially uniform intensity distribution over a particular aperture angle with respect to the permanent conductor, it can be readily calculated on the basis of optical principles that a portion of the light emitted within that aperture angle is reflected directly within the permanent conductor. This provides a stationary background independent of refractive index changes of the fuel mixture for a photoreceiver located at the opposite end of the permanent conductor from the light source. Another portion of the light is lost through direct transmission through the fuel mixture (also independent of the refractive index of the fuel mixture). The remaining portion of the light is reflected at the interface of the permanent conductor with the fuel mixture, and this reflection is dependent on the refractive index of the mixture.
According to the law of Snellius, the angle at which the light hitting the border surface of two media is still completely reflected is a function of the refractive indices of the two media. For example, van der Weide et al. describe evaluations of binary mixtures comprising ethanol (n=1.36) and gasoline (n=1.43). Using a glass rod (n=1.52) as a permanent conductor, van der Weide et al. calculated that a 100% ethanol composition would provide reflection up to a 27.degree. angle of incidence at the interface between the permanent conductor and the liquid medium, whereas a 100% gasoline composition would reflect only up to a 20.degree. angle. Using a round glass rod fitted at one end with a light emitting diode (650 nm) and a photoreceiver of the corresponding wavelength as a detector at the other, van der Weide et al. were able to confirm a correlation between the optical output voltage determined at the detector and the ethanol/gasoline ratio of the fuel mixture in which the central portion of the rod was immersed.
Unfortunately, the refractive indices of the liquids under consideration have a significant temperature sensitivity. Thus, over a temperature range of interest (e.g., -10.degree. C. to +50.degree. C.) the error in measurements obtained using the above-described system was unacceptably large (on the order of .+-.40%). Therefore, van der Weide et al. describe a further modification, wherein a second photoreceiver was fitted at the first end of the glass rod to monitor the amount of light transmission from the light source. A thermic connection between the two photoreceivers was made to achieve a thermic balance. As the light intensity registered by the second photoresistor depends on the temperature, the current was corrected for temperature over the range of -10.degree. C. to +60.degree. C.
Although such an arrangement does substantially reduce errors in the system based on variations in temperature, it has subsequently been determined by independent evaluation of data obtained in this manner using known compositions that such a system providing a single reading leads to calculations of fuel values with a margin of error on the order of .+-.10%. This is an unacceptably large margin, particularly in the context of supplying a suitable air/fuel ratio to, e.g., an internal combustion engine. For example, automobile engines have a fairly narrow range of air/fuel ratios at which they run acceptably; if the objective is optimizing the composition of vehicle emissions, the operating range is even narrower. This underscores the need for higher accuracy in the determination of fuel composition, particularly as more complex fuel mixtures are employed.
The devices for monitoring oxygenate levels in fuel compositions such as described above have heretofore been useful primarily to measure the degree of dilution of gasoline by oxygenates in general; in other words, a nominal gasoline concentration is obtained for the mixture. Such systems have a number of significant inherent drawbacks. For example, these devices sense water as if it were an oxygenated fuel; because water has no fuel value per se, the usefulness of such devices for making appropriate adjustments in fuel flow is limited with respect to fuels containing any significant water content.
Moreover, the known monitoring devices do not differentiate the various oxygenates with some fuel value from one another, and thus have utility primarily with respect to predetermined binary mixtures. Such differentiation among oxygenates is also of practical significance, as the materials available in the marketplace contain many different oxygenates and the required adjustments to fuel flow should be different for each oxygenate depending on its fuel value.
Finally, the various monitoring devices heretofore available do not make any adjustments for the varying aromatics content of fuels. As the commercially available fuels may vary significantly with respect to aromatics content, the ability to evaluate the amount of aromatics present in the fuel and to make corresponding adjustments in the calculated fuel value would be advantageous.
It is therefore an object of the present invention to provide an apparatus for determining the composition of a fuel mixture comprising gasoline and oxygenates with greater accuracy that has heretofore been possible. In particular, it is an object of the present invention to permit a rapid and accurate discrimination between oxygenates with no fuel value (e.g., water) and those with some fuel value, as well as to enable determination of the relative composition of the oxygenates component of the fuel mixture. In addition, it is a further object of the invention to provide methods and apparatus for determining the composition of fuel mixtures using low cost components which have low power consumption and which may conveniently be mounted in existing systems to enable fuel component stoichiometries to be determined in flexible fuel vehicles (e.g., automobiles and military vehicles) wherein such determinations of fuel composition would be of particular advantage, as well as in bulk fuel storage and transport facilities (for example, gasoline terminals and distribution systems).